It’s All About Leaves and Roots

The corn and soybean crops are mostly planted and mostly up in Illinois, even where replanting was needed due to poor conditions. It’s been a good planting season for corn in most of Illinois and a less favorable one for soybean, with cool, damp weather in mid-May and conditions ranging from too dry to too wet. Soybean replanting percentages were much higher than normal in some areas. About the only positive part of this is that replanted soybeans have ended up not too far behind fields that were not replanted, as the latter struggled with the weather and soil conditions.

Most areas have received rainfall in the past week, and growing degree-day (GDD) accumulations since May 1 are fairly close to long-term averages. Corn that was planted in early April received 150 to 250 GDDs before May 1, and as a result it is ahead of normal in its development. In central and southern Illinois, early-planted corn is at V10 to V12 and ranges from waist high to chest high. Stands are very good in most cornfields.

Soybean is coming along after the late start, with the fields planted in early May now at about V3, which means three fully expanded trifoliate leaves (those with three leaflets, as opposed to the bottom leaves on the stem, which have only a single leaflet). There is some debate about how to tell when a leaflet is fully expanded, but for practical purposes we can take it as the point when the leaf is at least as large as the leaf below it. That works for the first half of the season, when mature leaf size tends to increase as you move up the stem. After flowering, new leaves often do not get as large as older leaves, with the largest area per leaf found on leaves attached at about the midpoint of the stem.

Over the next month, the corn canopy will complete its growth in all but the latest-planted fields. Canopy formation is a critical process in crops, in that it sets the stage for successful flowering and grain filling. In corn, we often note when the canopy closes, which is the point when it appears that nearly all of the sunlight is falling on leaves rather than some getting through to the soil. With 30-inch rows, the canopy appears to close when the crop is about 30 to 36 inches tall, especially when there has been enough moisture that leaves are as wide and as long as usual. Moisture stress then makes leaves curl; hybrids with more upright leaf growth (though this is usually not nearly as apparent on the lower leaves as on the upper leaves) and wide rows make the canopy look less closed, and make canopy closure appear to happen later.

Is canopy closure as important as we think it is? It is clearly an advantage for the crop to be taking in as much sunlight as possible as early in the season as possible; so, yes, rapid development of leaf area is important. The longest day of the year is at the summer solstice, which occurs next Wednesday, June 21. On that day (if it’s not cloudy), the crop receives more sunlight energy than on any other day, and this does the crop good only to the extent that it has leaf area to intercept the light. If we had a way to manage it, having grain fill taking place in late June would be even better than having only leaves and stalks and roots growing, but at least the "factory" needed to produce a lot of kernels and to fill them later is being formed, and larger, heavier plants at the time of pollination tend to produce higher yields.

While our informal method of assessing leaf area is to assume the canopy is complete once the row middles seem to be covered by leaf area, walking out into the crop tells a different story. When we do that, we will find a considerable amount of light hitting the soil surface instead of leaves, even two or three weeks after the canopy appeared to close. Leaf area is not completely formed until about the time of silking, or when the crop reaches its maximum height. Thus narrow rows, which seem to close their canopy earlier than wider rows, might be taking in only a little more sunlight in mid-June than are wider rows. If a streak of sunlight is still visible around noon by the time the crop pollinates, though, this is a good indication that the rows are probably too wide to produce maximum yield. This is also affected by hybrid, especially plant size, and by growing conditions in June. Periods of dry weather reduce stem growth and final height, and they can also reduce the size of individual leaves, leaving the canopy less complete.

The formal way that we measure leaf area is by taking leaves off the plant and running them through an area meter, or by using a device that measures leaves while they are still on the plant. Because we are interested more in the leaf area of the whole plant community than in the area of individual plants, we usually convert the leaf area into a leaf area index (LAI), which is the leaf area per plant divided by the amount of ground area that each plant occupies. So, if we measure 6 square feet of leaf area from a plant and the plant occupies 1.5 square feet of land area (that would mean a plant population of about 29,000 plants per acre), then the LAI is 6/1.5 = 4. An LAI of 4 is a good, healthy number for corn and usually indicates a crop that can intercept at least 97% of the sunlight that falls on the crop. Of course, the leaf area has to be healthyleaves with disease or inadequate nutrients may still intercept light well, but they cannot use the light as well as healthy, dark green leaves can.

Roots are also viewed as critical to the success of vegetative development, and rightly so. As we found out during the very dry June in 2005, a good root system is capable of maintaining a good supply of water to the plant almost completely from the water stored in the soil. Unlike the canopy, though, root system size and health are very difficult to evaluate. Short of digging up root systems, the only way to assess their effectiveness is to note how the aboveground part of the plant responds to periods of low rainfall. Much more stress has been noted during dry June weather in some years, very little in others. The only reasonable way to explain this is as differences in the size and depth of the root system.

We think that dry June weather is often very helpful in helping roots reach their maximum effectiveness. This is both because fewer diseases develop when surface soils are dry and because dry surface soils mean less root growth near the surface but increased root growth deeper in the soil where there is more water available. Roots cannot grow into dry soil, but as long as the leaves of the plant are healthy and supplied with enough water, the supply of energy to the roots will continue. This energy (sugars, mostly) will be diverted to those roots where there is enough water to enable them to grow and to take up nutrients. Dry weather also means more sunshine, which helps crops grow.

Is it the case, then, that the bigger the root system the better? No. Roots have an optimum size, where the benefit they provide to the plant is matched by the cost to the plant of growing and maintaining the root system. Seldom if ever is this optimum actually reached, but ideally there should be enough photosynthesis during vegetative growth to result in rapid growth of both roots and aboveground plant parts, with roots ending up larger than normal but still in a favorable proportion to the tops. Wet June weather will often favor top growth over root growth, since roots do not grow well into very wet soils. Dry June weather results in a better balance, unless it’s so dry that top growth is reduced, in which case roots will start to suffer as well.

Healthy root systems do a great deal to reduce stress during pollination, which in turn goes a long way in setting the course for high yields. So far in 2006, the corn crop is doing quite well on both ends (tops and roots), and we hope this balance can continue. As water movement through plants continues to build along with the canopy, though, demands on soil water will start to deplete that supply, and we will need some rainfall to make up the difference. Few areas in Illinois are critical yet, but water loss rates are approaching an inch per week now, and most soils will need some help from rain within the next three weeks if maximum crop growth rates are to be maintained.--Emerson Nafziger